Metal halide lamp, metal halide lamp operating device, and headlamp device for automobiles

ABSTRACT

The present invention relates to a metal halide lamp substantially containing no mercury, a metal halide lamp lighting device using the same and an automotive headlamp apparatus using the same. And object of the invention is to provide such products in which a rapid rising of luminous flux is achieved. A metal halide lamp (MHL) according to the present invention has: a discharge vessel having an inner volume of C (cc) and having a pair of electrodes ( 2 ), ( 2 ) sealed in a hermetic vessel ( 1 a) at opposite ends of a discharge space ( 1 a) in the hermetic vessel ( 1 a) at a distance of 5 mm or less; and a discharge medium containing xenon gas at 3 atmospheres or higher, a halide of sodium Na, and at least one of halides of scandium Sc and rare earth metals, the melting point of the halides being T (K), in which a lamp power in a stable state is 50 W or lower, and the formula (1) is satisfied: 
 
( H/C )×[ R /( T /500) 6 ]&lt;3.11  (1), 
where the amount of the halide deposited on the electrodes when the lamp is off is H (mg), and the ratio of a maximum lamp power at the start of lighting to the lamp power in the stable state is R.

TECHNICAL FIELD

The present invention relates to a metal halide lamp substantiallycontaining no mercury, a metal halide lamp lighting device using thesame, and an automotive headlamp apparatus using the same.

BACKGROUND ART

Metal halide lamps which have a hermetic vessel with a pair of opposingelectrodes containing an inert gas, a halide of a light-emitting metaland mercury in the vessel are used widely because of their relativelyhigh efficiency and good color rendering. Such metal halide lamps havebecome widely used also as automotive headlamps. Including those used asthe automotive headlamps, the metal halide lamps currently in practicaluse essentially uses mercury (conveniently referred to as amercury-containing lamp, hereinafter). In Japanese Patent Laid-Open No.2-7347, there is described an exemplary specification of a metal halidelamp used as the automotive headlamp, which specifies that about 2-15 mgof mercury has to be sealed. Besides, in Japanese Patent Laid-Open No.59-111244, there is described a discharge lamp, that is, a metal halidelamp, suitable for the automotive headlamp which contains mercury in apredetermined amount prescribed. According to the description, when thismetal halide lamp operates in a horizontal position, the discharge arcshrinks to be at least substantially linear, and the metal halide lampis efficient.

However, nowadays environmental issues are becoming serious, and in theilluminating industry, it is considered highly important to reduce oreven eliminate mercury in lamps, which applies a significant load to theenvironment.

To address this problem, several approaches to eliminate mercury in themetal halide lamp have been already proposed. For example, the inventorshave made the inventions described in Japanese Patent No. 2982198 andJapanese Patent Laid-Open Nos. 6-84496 and 11-238488. The firstinvention is a metal halide lamp which has a halide of scandium Sc or arare earth metal and an inert gas sealed therein and is controllablyturned on and off by a pulse current. The second invention is a metalhalide lamp which contains a discharge medium constituted by a metalhalide and an inert gas and thus has a less variable colorcharacteristic over a wide input range, thereby being capable of dimmingillumination. The third invention is a metal halide lamp which isimproved in electrical characteristic by containing, in addition to afirst metal halide, which is a primary light-emitting material, a secondmetal halide, which has a high vapor pressure and is hard to emit light.

Furthermore, in Japanese Patent Laid-Open No. 11-307048, there isdescribed a metal halide lamp which avoids blackening due to scatteringof the electrodes by containing, in addition to the halides of scandiumSc and sodium Na, the halides of yttrium Y and indium In as third metalhalides which have a vapor pressure of 1×10⁻⁵ atmospheres in operationand whose metals themselves are ionized at 5 -10 eV. The metal halidelamp according to the invention disclosed in this document is describedas having any luminous flux and chromaticity range required for theautomotive headlamp.

FIG. 13 is an enlarged view of an essential part of a conventionalmercury-containing lamp turned off. In this drawing, reference numeral101 denotes a hermetic vessel, reference numeral 102 denotes anelectrode, and reference numeral 103 denotes a halide.

The discharge vessel 101 comprises a hermetic vessel 101 a and a pair ofelectrodes 101 b, and a significant amount of halide 104 is deposited onshaft parts of the electrodes 101 b.

In the case of a mercury-containing lamp for an automotive headlamp,xenon primarily emits light immediately after the lamp is turned on, andthen, mercury is vaporized quickly and abruptly to begin to emit light.Since the efficiency of light emission of mercury is several timeshigher than that of xenon, 80% or higher of a rated luminous flux isachieved 4 seconds after the turn-on of the lamp, and thus, relativelyrapid rising of luminous flux is achieved. The luminous flux describedabove can be attained by inputting a power about twice as high as arated lamp power, that is, a lamp power in a stable state immediatelyafter the turn-on of the lamp. A maximum lamp current flows onlyimmediately after the turn-on of the lamp, and the lamp currentdecreases abruptly in 1-2 seconds after the turn-on and is equal to orlower than a half of the maximum current after 4 seconds.

On the other hand, in the case of a metal halide lamp substantiallycontaining no mercury (conveniently referred to as a mercury-free lamphereinafter) used as an automotive headlamp, xenon emits lightimmediately after the lamp is turned on, as with the mercury-containinglamp. Then, however, the halide is not sufficiently vaporized before thetemperature thereof rises to about 400-600° C., and this takes about 4seconds after the turn-on of the lamp is started. Therefore, xenoncontinues to emit light for the meanwhile. Thus, there is a problem thatthe rising of luminous flux of the mercury-free lamp achieved with alamp power is inferior to that of the mercury-containing lamp, and acurrent close to the maximum lamp current has to be flown for about 4seconds after the lamp is turned on, as shown in FIG. 1.

FIG. 1 is a graph showing variations of lamp currents of a mercury-freelamp and a mercury-containing lamp after the lamps are turned on. FIG. 2is a graph showing variations of electrode temperatures thereof, andFIG. 3 is a graph showing variations of vapor pressures thereof. In thedrawings, the horizontal axis indicates time (second), and the verticalaxis in FIG. 1 indicates lamp current, the vertical axis in FIG. 2indicates electrode temperature, and FIG. 3 indicates vapor pressure ofthe halide and mercury, all the vertical axes indicating relativevalues. In the drawings, the curves A are for the mercury-free lamp, andthe curves B are for the mercury-containing lamp.

As described above, the mercury-free lamp used as an automotive headlampis temporarily supplied with a relatively high lamp current when it isturned on to provide a rapid rising of luminous flux. Thus, as shown inFIG. 4, it emits an instantaneous intense light exhibiting orange forabout 0.2 to 2 seconds after the turn-on.

FIG. 4 is a graph showing rising characteristics of luminous flux of aconventional mercury-free lamp and a mercury-free lamp according to thepresent invention at the time of turn-on. In this drawing, thehorizontal axis indicates time (after turn-on) (second), and thevertical axis indicates rising ratio of luminous flux (%). In thisdrawing, the curve C is for the present invention, and the curve D isfor the conventional mercury-free lamp such as one shown in FIG. 13. Themercury-free lamp according to the present invention will be describedlater.

The instantaneous and short-duration intense light emitted immediatelyafter the turn-on of the lamp has a luminance several times higher thanthe light emitted in a stable state and often exhibits orange due to Na,which is easy to emit light. However, the light may exhibit variouscolors because it may contain light emitted by Sc or another metal. Whenthe metal halide lamp is used as an automotive headlamp, such lightemission is not preferred in terms of safety, and thus, has to besuppressed.

To the contrary, in the mercury-containing lamp, the intense lightemission immediately after the turn-on thereof described above does notoccur, or, if any, occurs for an extremely short time, leading to nopractical problem.

An object of the present invention is to provide a metal halide lampsuitable for use as an automotive headlamp which substantially containsno mercury out of consideration to the environment and is improved inrising of luminous flux, a metal halide lamp lighting device using thesame and an automotive headlamp apparatus using the same.

Another object of the present invention is to provide the productsdescribed above in which an instantaneous intense light emission in 2seconds after the turn-on is suppressed.

Another object of the present invention is to provide the productsdescribed above which are improved in efficiency of light emissionwithout loss of life.

Another object of the present invention is to provide the productsdescribed above in which discharge is stabilized.

Another object of the present invention is to provide the. productsdescribed above having a desired light distribution.

Another object of the present invention is to provide the productsdescribed above which are improved in reliability.

Another object of the present invention is to provide the productsdescribed above which are improved in reliability by reducing wear ofelectrodes to suppress the occurrence of various defects due to the wearof the electrodes.

DISCLOSURE OF THE INVENTION

A metal halide lamp according to the embodiment described in claim 1 ischaracterized in that the metal halide lamp comprises: a dischargevessel having a hermetic vessel which is fire resistant and translucentand has a discharge space therein, and a pair of electrodes provided atopposite ends of the discharge space in the hermetic vessel with facingeach other at a distance of 5 mm or less, the inner volume of thehermetic vessel being C in terms of cc; and a discharge mediumsubstantially containing no mercury, sealed in the hermetic vessel, andcontaining xenon at 3 atmospheres or higher, a halide of sodium Na, andat least one of halides of scandium Sc and rare earth metals, themelting point of the halides being T in terms of K, in a stable state,the metal halide lamp is kept on with a. lamp power of 50 W or lower,and the formula (1) is satisfied:(H/C)×[R/(T/500)⁶]<3.11  (1),

where the amount of the halide deposited on the electrodes when the lampis off is denoted by H in terms of mg, and the ratio of a maximum lamppower at the start of lighting to the lamp power in the stable state isdenoted by R.

Terms used in this embodiment and the embodiments described later havedefinitions and technical meanings as follows unless otherwisespecified. In the present invention, the discharge vessel, the dischargemedium and the like are essential components. In the following, eachcomponent will be described.

<Discharge Vessel>

The discharge vessel comprises a hermetic vessel and a pair ofelectrodes.

(Hermetic Vessel)

The hermetic vessel is fire resistant and translucent and has adischarge space formed therein. The words “fire resistance” mean thatthe hermetic vessel can adequately withstand a normal operatingtemperature of the metal halide lamp. Therefore, the hermetic vessel maybe made of any material as far as it has a fire resistance and can allowthe visible light in a desired wavelength range produced by discharge tobe emitted to the outside. For example, the hermetic vessel may be madeof a ceramic, such as quartz glass, translucent alumina and YAG, or asingle crystal thereof. As required, the inner surface of the hermeticvessel may be coated with a transparent film having a halogen resistanceor halide resistance, or may be modified.

The discharge space formed in the hermetic vessel preferably has anelongated shape. For example, it has a cylindrical, spheroidal orspindle shape.

Furthermore, a part of the hermetic vessel which surrounds the dischargespace can have a relatively high thickness. That is, a part of thehermetic vessel around the middle of the distance between the electrodescan be thicker than the end parts thereof. This enhances heat transferof the hermetic vessel, whereby the temperature of the halide adheringto the inner surface of the lower part and side part of the dischargespace of the hermetic vessel increases rapidly. Thus, a rapid rising ofluminous flux is attained.

(A Pair of Electrodes)

The pair of electrodes is sealed at opposite ends of the discharge spacein the hermetic vessel with facing each other at a distance of 5 mm orless. The electrodes may be made of tungsten, doped tungsten, rhenium, arhenium/tungsten alloy or the like, have an elongated rod shape, and besupported with the base end parts thereof being embedded in the ends ofthe hermetic vessel and the tip end parts thereof protruding into thehermetic vessel. In the case of a metal halide lamp for an automotiveheadlamp, as desired, the electrodes can have a maximum-diameter sectionthicker than the shaft part thereof at a short distance from the tipends thereof. That is, the lamp is turned on and off highly frequently,and a lamp current higher than that in a stable state flows at the startof lighting. If the diameter of the electrodes is entirely increased, acrack is likely to occur in the parts of the hermetic vessel in contactwith the shaft parts of the electrodes because the parts are subject tothermal stress each time the lamp is turned on and off. If themaximum-diameter sections are provided near the tip ends of theelectrodes as described above, the shaft parts of the electrodes are notincreased in diameter, and therefore, a crack is hard to occur.

Furthermore, the electrodes may be configured for an alternating currentor direct current. If the lamp is operated by an alternating current,the electrodes of the pair have the same structure. If the lamp isoperated by a direct current, in general, the temperature of the anodeincreases rapidly. Thus, if the maximum-diameter section is formed onthe anode at a short distance from the tip end, the heat radiating areacan be increased, and thus, the anode can be ready for a frequent on/offoperation. To the contrary, the cathode may not have themaximum-diameter section.

Furthermore, the electrodes are supported by being embedded in thehermetic vessel and are externally supplied with power through aconductive means hermetically introduced into the hermetic vessel. Inthe case where the hermetic vessel is made of quartz glass, theconductive means may be a well-known sealed metal foil. Specifically, asthe sealed metal foil, a foil of molybdenum or the like is hermeticallyembedded in the sealing part of the hermetic vessel with one end beingwelded to the base end of the electrode and the other end being weldedto the tip end of the externally introduced line. The sealed metal foilcan be hermetically embedded in a known sealing manner, such aschip-less decompression sealing or pinch sealing.

<Discharge Medium>

The discharge medium contains a halide and an inert gas andsubstantially contains no mercury.

(Halide)

The halides include, as halides of light-emitting metals, at least ahalide of sodium Na and at least one of halides of scandium Sc and rareearth metals. Preferably, the halide of the light-emitting metalconstitutes a first halide, and a second metal halide described later isadded thereto.

Sodium Na, scandium Sc and rare earth metals described above are highlyefficient light emitting material, and sodium Na and/or scandium Sc andrare earth metals is a primary light-emitting metal in this invention.However, as required, a halide of another light-emitting metal, such asIn, can be added for color adjustment, for example. In the case where ahalide of Zn is used as the second metal halide described later, Znprovides color adjustment because Zn emits blue light.

Now, a case where the second metal halide is added will be described.The second metal halide is characterized by a high vapor pressure. Thus,it is sealed as a discharge medium primarily to provide a lamp voltage.The second metal halide is preferably one or more selected among fromhalides of Mg, Co, Cr, Zn, Mn, Sb, Re, Ga, Sn, Fe, Al, Ti, Zr And Hf. Byusing the second metal halide instead of mercury, a lamp voltage ofabout 25 to 70 V can be achieved in the metal halide lamp which has theelectrodes at a distance of 5 mm or less and has a lamp power of 50 W orlower.

Besides the relatively high vapor pressure, the second metal halide hasa characteristic that it emits a relatively little visible light.However, this is not very important in this invention.

Thus, sealing the second metal halide in addition to the first metalhalide in the hermetic vessel can increase the lamp voltage so as tofall within a desired range, and therefore, a required lamp power can beinput with a relatively low lamp current.

Now, a halogen of a halide will be described. That is, in terms ofreactivity, iodine is the most suitable. At least the primarylight-emitting metal described above is sealed in the hermetic vessel inthe form of an iodide. However, as required, different compounds ofhalogens, for example iodide and bromide, may be used together.

Furthermore, the halide is sealed in the hermetic vessel in an excessiveamount, and an excess of the halide, which is not evaporated, remains asthe liquid phase and adheres to the inner wall of the bottom part andside part of the discharge space when the lamp is on.

(Xenon)

Xenon gas serves as a starting gas and a buffer gas and serves also todominantly emit light immediately after the lamp is turned on. Thepressure of the sealed xenon gas is 3 atmospheres or higher, preferablyis at 5 atmospheres or higher and most preferably falls within a rangefrom 9 to 16 atmospheres. Therefore, even if the vapor pressure of thehalides is low for a several seconds after the lamp is turned on, thelamp voltage of the metal halide lamp can be maintained as high aspossible. Thus, a higher lamp power can be provided with a same lampcurrent, and the rising characteristics of luminous flux can beimproved. The good rising characteristics of luminous flux, which areadvantageous for any use of the lamp, are extremely importantparticularly in applications of automotive headlamp, liquid-crystalprojector and the like.

(Mercury)

The words “substantially contain no mercury” in this invention mean thatmercury is not sealed at all or that mercury may exist in an amount ofless than 2 mg/cc of the inner volume of the hermetic vessel, preferably1 mg/cc of the inner volume of the hermetic vessel or less. However,from an environmental point of view, it is desirable that no mercury issealed. If the electrical characteristics of the discharge lamp aremaintained by a mercury vapor as in the prior art, the mercury has to besealed in the hermetic vessel in an amount of 20 to 40 mg/cc, possibly50 mg/cc, of the inner volume of the hermetic vessel in the case of ashort arc type metal halide lamp. Compared with this, the amount ofmercury used in this invention is significantly reduced. <Lamp PowerImmediately After the Lamp is Turned on>

According to this invention, a period in which a power two or more timeshigher than the lamp power in the stable state is input is providedimmediately after the lamp is turned on. This makes the rising ofluminous flux more rapid. Preferably, a power 2.5 to 4 times higher thanthe lamp power in the stable state is input, and most preferably, apower 3 times higher than that is input. The input lamp power can beadjusted mainly in a lighting circuit.

<Amount of Halides Deposited on the Electrodes When the Lamp is off>

The “amount of halides deposited on the electrodes” when the lamp is offrefers to the amount of halides deposited on the peripheries of theelectrodes when the lamp is off. The “peripheries of the electrodes”refer to the areas within a 0.2 mm radius of the shaft parts of theelectrodes. Thus, the amount of halides deposited on the electrodesrefers to the amount of halides deposited in the areas within a 0.2 mmradius of the shaft parts of the electrodes.

In this invention, the amount of halides deposited on the electrodeswhen the lamp is off is to be reduced. As described later, the degree ofreduction of the amount of halides deposited on the electrodes when lampis off affects the magnitude of the ratio of an instantaneous maximumluminous flux within 2 seconds after the turn-on to a luminous flux inthe stable state. That is, if the amount of halides deposited on theelectrodes when the lamp is off is adequately reduced, the ratio of theinstantaneous maximum luminous flux within 2 seconds after the turn-onto the luminous flux in the stable state is 110% or lower. According tothis invention, in the case where the metal halide lamp is used as anautomotive headlamp, the amount of halides deposited on the electrodeswhen the lamp is off is about 0.18 mg or less.

While a measure to reduce the amount of halides deposited on theelectrodes when the lamp is off as described above is not limited to aparticular one, one or more of measures described below may be used.

-   1. The parts of the electrodes protruding into the discharge space    in the hermetic vessel are reduced. This allows heat of the    electrodes to be more readily transferred to root parts thereof, and    therefore, the temperature at the root parts increases. Thus, the    amount of halides deposited on the electrodes is reduced.-   2. Any wedge-shaped or pocket-like clearance is prevented from being    formed at the parts of the hermetic vessel where the electrodes are    embedded. This inhibits the halides from being deposited at the root    parts of the electrodes, and thus, the amount of halides deposited    on the electrodes is reduced.-   3. The walls of the parts of the hermetic vessel which surround the    electrodes provided at opposite ends of the discharge space are    brought close to the respective electrodes. This increases the    temperature of the root parts of the electrodes, and thus, the    amount of halides deposited on the electrodes is reduced.

<Formula (1)>

This invention is to suppress the occurrence of an instantaneous intenselight emission within 2 seconds after the lamp is turned on bysatisfying the formula (1) having parameters of C(cc), which is theinner volume of the hermetic vessel, T(K), which is the melting point ofthe halides, H(mg), which is the amount of halides deposited on theelectrodes when the lamp is off, and R, which is the ratio of themaximum lamp power at the start of lighting to the lamp power in thestable state. The formula (1) is experimentally derived, and the valuesof the parameters are absolute ones.

Therefore, as the ratio R of the maximum lamp power at the start oflighting to the lamp power in the stable state, that is, the maximumlamp power at the start of lighting increases, the temperature of theelectrodes at the start of lighting increases, and the input powerincreases. Thus, the amount of light emitted tends to increase, and theinstantaneous light emitted within 2 seconds after the turn-on tends tobe more intense. In addition, as the melting point T of the halidesdecreases, the rate of evaporation of the halides increases, the amountof the halides deposited on the electrodes increases, and theinstantaneous intense light is more likely to be emitted.

The formula (1) represents a relationship among the above parameters.

<Lamp Power>

The lamp power is a power supplied to the metal halide lamp. Accordingto this invention, it is 50 W or lower in a steady state, that is, astable state. This means that the lamp is a small metal halide lamp.

<Other Components in this Invention>

The following components are not essential in the metal halide lampaccording to this embodiment and other embodiments. However, selectivelyadding any of these components to the metal halide lamp can enhance theperformance and the function thereof.

1. Outer Jacket

The outer jacket houses the discharge vessel therein. The outer jacketcan block ultraviolet rays from being emitted from the discharge vesselto the outside, maintain the temperature of the discharge vessel,mechanically protect the discharge vessel or adapt the discharge vesselfor any desired purpose. As required, the outer jacket may behermetically sealed from the outside air or may have air or an inert gasat an atmospheric or reduced pressure sealed therein. Furthermore, asrequired, it may be communicated with the outside air.

2. Cap

The cap serves to connect the metal halide lamp to the lighting circuitor mechanically support the metal halide lamp at a predeterminedposition.

3. Igniter

The igniter is to produce a high pulsed voltage and apply the voltage tothe metal halide lamp to promote starting of the metal halide lamp. Asrequired, it maybe integrated with the metal halide lamp by being housedinside the cap.

4. Start Assistant Conductor

The start assistant conductor is to increase an electric field strengthin the vicinity of the electrodes, thereby facilitating starting of themetal halide lamp. One end of the start assistant conductor is connectedto a part at the same potential as one electrode, and the other endthereof is disposed on a region of the outer surface of the dischargevessel in the vicinity of the other electrode.

<Operation of the Invention>

The inventors have observed that, in a mercury-free lamp, there is amixture of halides deposited on the shaft parts of the electrodes afterthe lamp is turned off, and the mixture in the liquid phase flows to thetip ends of the electrodes, and also found that the amount of thehalides that flow to the tip ends of the electrodes depends on theamount of the halides deposited on the electrodes. It can be consideredthat this is because the melting point of the mixture of the first metalhalide and the second metal halide sealed together is lower than that ofthe first metal halide, and thus, the time for the mixture of thehalides in the liquid phase to be solidified after the lamp is turnedoff is longer than that for the first halide.

Since the halides have a lower melting point as described above, thehalides have an increased evaporation rate. Therefore, at the start oflighting, the halides having moved to the tip ends of the electrodes areevaporated instantaneously and emit instantaneous light. At this time,Na or the like, which is likely to emit light, emits intense light. Inaddition, at the start of lighting, a high lamp power is continuouslyinput, and therefore, the electrodes have a relatively high temperature,which also promotes the instantaneous light emission at the start oflighting.

In addition, through numerous trials and detailed observations, theinventors have found that the amount of the halides that move to the tipends of the electrodes depends on the amount of the halides deposited onthe electrodes. Specifically, as the amount of the halides deposited onthe electrodes increases, the amount of the halides that move to the tipends of the electrodes also increases, resulting in more intenseinstantaneous light emission at the start of lighting.

According to the invention, as far as the formula (1) is satisfied, theinstantaneous intense light emission for 2 seconds after the turn-on canadequately suppressed. However, if the formula (1) is not satisfied, theinstantaneous intense light emission for 2 seconds after the turn-oncannot be suppressed adequately.

In addition, according to the invention, 60% or higher of the luminousflux in the stable state can be readily achieved 4 seconds after thelamp is turned on. Thus, the specification for the automotive headlampis met. Thus, the instantaneous light emission for 2 seconds after thelamp is turned on is not practically problematic. It is preferred that60 to 110% of the luminous flux in the stable state is achieved. In sucha case, a good rising of luminous flux can be achieved, and therequirement that 60% or higher of the luminous flux in the stable statehas to be achieved 4 seconds after the lamp is turned on, which isspecified for the metal halide lamp for automotive headlamp use, isreadily satisfied, and a smooth change of luminous flux can be achievedduring a period from 2 seconds after the lamp is turned on until 4seconds after the turn-on. Furthermore, if the luminous flux is 105% orlower of that in the stable state, it is not visually perceived asintense light. Here, the rapid rising of luminous flux described aboveis useful in applications other than the automotive headlamp.

A metal halide lamp according to the embodiment described in claim 2 ischaracterized in that the metal halide lamp comprises: a dischargevessel having a hermetic vessel which is fire resistant and translucentand has a discharge space therein, and a pair of electrodes sealed atopposite ends of the discharge space in the hermetic vessel with facingeach other at a distance of 5 mm or less; and a discharge mediumsubstantially containing no mercury, sealed in the hermetic vessel, andcontaining a halide of a light-emitting metal and an inert gas, and in astable state, the metal halide lamp is kept on with a lamp power of 50 Wor lower, during a period of 10 seconds after the lamp is turned on, alamp power 2.2 or more times higher than the lamp power in the stablestate is supplied to the lamp, 60% of the luminous flux in the stablestate is achieved 4 seconds after the lamp is turned on, and the formula(2) is satisfied:5<(L _(A−H))³ ×C _(T) /B _(W)<28  (2),

where the lamp power in the stable state is B_(W) (W), a minimum lengthbetween a point in an arc having a maximum luminance and a pool of thedischarge medium in the liquid phase is L_(A−H) (mm), and the mass ofthe discharge space section of the hermetic vessel is C_(T) (mg).

According to this embodiment, a metal halide lamp is prescribed which isturned on and off as an automotive headlamp and arranged to provide arapid rising of luminous flux. Except for the points described above,the hermetic vessel and electrodes of the discharge vessel and thedischarge medium in this embodiment may be the same as those describedconcerning the embodiment of claim 1.

In the stable state, the metal halide lamp is kept on with a power of 50W or lower, and during a period of 10 seconds after the lamp is turnedon, a lamp power 2.2 or more (preferably 2.5) times higher than the lamppower in the stable state is supplied to the lamp from a lightingcircuit. Thus, the metal halide lamp provides 60% or more of theluminous flux in the stable state 4 seconds after the lamp is turned on.Therefore, according to this embodiment, the metal halide lamp is turnedon as desired by a metal halide lamp lighting device, which is animplementation of the metal halide lamp that cooperates with thelighting circuit.

The minimum length L_(A−H) (mm) between a point in an arc having amaximum luminance and a pool of the discharge medium in the liquid phaseis measured at the middle between the electrodes. The point in an archaving a maximum luminance is determined using a luminance meter. Thepool of the discharge medium can be seen by observing laterally thedischarge vessel when the lamp is on. The pool of the discharge mediumis not significantly changed when the lamp is off. The words “pool ofthe discharge medium” mainly refers to an excess of halides in theliquid phase adhering to the inner wall of the discharge space.

The mass C_(T) (mg) of the discharge space section of the hermeticvessel refers to the mass of the sheath section of the hermetic vesselthat surrounds the discharge space, excluding the mass of the sealingparts connected to the sheath section. The discontinuities between thesheath section and the sealing parts connected thereto can be recognizedas boundaries.

The inert gas may be one or more of xenon, krypton, argon and neon.While the pressure of the inert gas is not limited to a particularvalue, it is preferably 3 atmospheres or higher, more preferably 5atmospheres or higher and, most preferably, 8 to 16 atmospheres.

The metal halide lamp according to this embodiment is arranged asdescribed above to address the evaporation of the discharge medium,which is inherent to the mercury-free lamp. Therefore, the arc isbrought close to the pool of the discharge medium, thereby making thetemperature of the discharge medium increase rapidly, and the mass ofthe hermetic vessel per power is reduced to decrease the thermalcapacity thereof. Thus, the rising of luminous flux is significantlyimproved.

If the metal halide lamp of this embodiment is implemented incombination with the arrangement described in claim 1, the metal halidelamp can be more practical.

The metal halide lamp according to the embodiment described in claim 3is the metal halide lamp described in claim 2 that is furthercharacterized in that the paired electrodes each have an averagediameter of C_(E) (mm) in a section embedded in the hermetic vessel andhave a maximum-diameter section in a part protruding into the dischargespace, the average diameter of the protruding part being D_(E) (mm), andthe formulas (3) and (4) are satisfied:C_(E)<D_(E)   (3), andD _(E) −C _(E)>0.05  (4).

According to this embodiment, an arrangement is prescribed which hasrapid rising of luminous flux due to the improvement of the electrodesand is improved in efficiency and life. Specifically, the electrodes aremade of tungsten, doped tungsten, rhenium, a rhenium/tungsten alloy orthe like. Since these materials have thermal conductivities remarkablyhigher than the material of the hermetic vessel, such as quartz glass,if the electrodes are configured as described above, the shaft partsthereof are relatively thin, and the heat transfer from the electrodesto the sealing parts of the hermetic vessel is reduced. As a result, thetemperature of the discharge vessel rises more rapidly, more rapidrising of luminous flux is achieved, and the efficiency is improved. Inaddition, the temperature of the parts of the electrodes embedded in thehermetic vessel is reduced, and therefore, the reaction of the sealedmetal foils in the sealing parts with the halides is reduced, so thatthe life of the metal halide lamp is extended.

Since the parts of the electrodes protruding into the discharge spaceeach have the maximum-diameter section which is wider than that of themetal halide lamp containing mercury, and therefore, the electrodes havea higher thermal capacity, the tip ends of the electrodes are not molteneven if a high lamp current flows for a relatively long time at thestart of lighting.

If the metal halide lamp of this embodiment is implemented incombination with the arrangement described in claim 1, the metal halidelamp can be more practical.

The maximum-diameter section of the electrode can be formed by mountinga coil of tungsten around the shaft part of the electrode, or formedintegrally with the shaft part by trimming a thick tungsten rod.

The metal halide lamp according to the embodiment described in claim 4is the metal halide lamp described in claim 2 that is furthercharacterized in that the maximum diameter of the part of each of thepaired electrodes protruding into the discharge space is B_(E) (mm), theaverage diameter for the distal 10% thereof is A_(E) (mm), and theformula (5) is satisfied:A_(E)<B_(E)  (5).

According to this embodiment, an arrangement is prescribed which hasimproved electrodes and thus is improved in stability of discharge. Ifthe electrodes are configured as described above, the electrodes havethe maximum diameter at a short distance from the tip ends thereof.Therefore, the shaft parts of the electrodes are relatively thin, andthe temperature thereof is increased, so that the thermionic emission atthe tip ends is improved, and the discharge is stabilized. Thus,extinction of the arc or occurrence of a luminance flicker can beprevented. In addition, since the maximum-diameter sections are formedat a short distance from the tip ends of the electrodes, the electrodeshave a higher thermal capacity, the tip ends of the electrodes are notmolten even if a high lamp current flows for a relatively long time atthe start of lighting.

The maximum-diameter section of the electrode can be formed by mountinga coil of tungsten around the shaft part, or formed integrally with theshaft part by trimming a thick tungsten rod.

The inert gas may be one or more of xenon, krypton, argon and neon.However, xenon is preferably used. While the pressure of the inert gasis not limited to a particular value, it is preferably 3 atmospheres orhigher, more preferably 5 atmospheres or higher and, most preferably, 8to 16 atmospheres.

If the metal halide lamp of this embodiment is implemented incombination with the arrangement described in claim 1, the metal halidelamp can be more practical.

The metal halide lamp according to the embodiment described in claim 5is the metal halide lamp described in claim 2 that is furthercharacterized in that the paired electrodes each have an averagediameter of C_(E) (mm) in a section embedded in the hermetic vessel, themaximum diameter of the part of each of the paired electrodes protrudinginto the discharge space is B_(E) (mm), the average diameter for thedistal 10% thereof is A_(E) (mm), the average diameter of the protrudingpart being D_(E) (mm), and the formulas (3) and (6) are satisfied:C_(E)<D_(E)  (3), andA _(E) <D _(E) <B _(E)  (6).

According to this embodiment, an arrangement is prescribed in whichdisplacement of a cathode spot is suppressed to prevent the lightdistribution characteristic from fluctuating, and the tip ends of theelectrodes are made less susceptible to damage. Arranged as describedabove, a cathode spot is formed. However, since the electrodes have thethin tip ends, the location where the cathode spot is formed is lessvariable, so that the light distribution characteristic is lesssusceptible to fluctuation.

In addition, since xenon gas is sealed at 8 to 16 atmospheres, thepressure of 16 atmospheres being preferred to avoid the risk ofbursting, a high lamp voltage can be achieved during the dischargecontributed only by xenon immediately after turn-on. Therefore, areduced maximum lamp current is enough to input a desired lamp power tothe lamp during this period, so that the electrodes can be thinner.Consequently, displacement of the cathode spot and, therefore,fluctuation of the light distribution are further suppressed.

Furthermore, since the electrodes have the maximum diameter sections ata short distance from the tip ends thereof, the electrodes have a higherthermal capacity, so that the heat dissipation is accelerated and thetemperature reduction is improved.

The metal halide lamp according to the embodiment described in claim 6is the metal halide lamp described in claim 2 that is furthercharacterized in that each of the paired electrodes has a large-diametersection at a short distance from the tip end, and an angle Q_(E) (°)between the axis of the electrode and a line drawn from a shoulder ofthe tip end to pass through an outermost point of the large-diametersection satisfies the formula (7):24≦Q _(E)≦43  (7).

Having the arrangement described above, this embodiment providessubstantially the same operation and advantage as those described inclaim 5.

In addition, the arc tends to be curved when heavy xenon is sealed at 8atmospheres or higher. However, according to this embodiment, even ifthe arc is curved significantly, the outermost point of thelarge-diameter section formed at a short distance from the tip end ofthe electrode lies within a range of the angle Q_(E), so that thecathode spot is prevented from being formed unwantedly at themaximum-diameter section. Here, the “outermost point” refers to a pointin the circumference of the large-diameter section with which the linedrawn from a shoulder of the tip end of the electrode first intersects.

When checking whether the outermost point of the large-diameter sectionof the electrode satisfies the abode-described condition, if the tip endof the electrode is semi-spherical or paraboloidal, the tip end isassumed to be planar to determine the shoulder.

A metal halide lamp according to the embodiment described in claim 7 ischaracterized in that the metal halide lamp comprises: a dischargevessel having a hermetic vessel which is made of quartz glass and has adischarge space therein, and a pair of electrodes provided at oppositeends of the discharge space in the hermetic vessel with facing eachother at a distance of 5 mm or less, the atom density ratio A (%) ofSiO₂ at the surface of the tip ends of the electrodes satisfying theformula (8):2.5<A<43  (8),

a discharge medium substantially containing no mercury, sealed in thehermetic vessel, and containing xenon gas at 3 atmospheres or higher andat least one of halides of sodium Na, scandium Sc and a rare earthmetal, in a stable state, the metal halide lamp is kept on with a lamppower of 50 W or lower, and a period in which a power two or more timeshigher than the lamp power in the stable state is input is providedimmediately after the lamp is turned on.

According to this embodiment, an arrangement is prescribed whichsubstantially uses no mercury out of consideration to the environment,provides for rapid rising of luminous flux, and is improved inreliability by reducing wear of the electrodes to suppress theoccurrence of various defects due to the wear of the electrodes. Exceptfor the points described above, the hermetic vessel and electrodes ofthe discharge vessel and the discharge medium in this embodiment may beselectively configured the same as those described in claim 1 and/orclaims 2 to 6.

The pair of electrodes are sealed in the hermetic vessel with facingeach other at the opposite ends of the discharge space and spaced apartfrom each other by 5 mm or less, and the atom density ratio A (%) ofSiO₂ at the surface of the tip ends of the electrodes satisfies theformula (8). Here, the atom density ratio A(%) of SiO₂ at the surface ofthe tip ends of the electrodes to a depth of several nanometers ismeasured with an XPS (X-ray diffractometer).

By the atom density ratio A(%) of SiO₂ satisfying the formula (8), theintended object that wear of the electrodes is reduced to suppress theoccurrence of various defects due to the wear of the electrodes, therebyimprove the reliability of the metal halide lamp is attained.

However, if the atom density ratio A(%) is higher than 43%, the wear ofthe electrodes becomes significant, whitening, blackening and/orincrease of the distance between the electrodes are beyond therespective acceptable levels, and the luminous flux maintenance factoris reduced accordingly. Therefore, such an atom density ratio is notacceptable. The whitening is caused by the scattered electrode materialreacting with quartz glass forming the translucent hermetic vessel. Theblackening is caused by the scattered electrode material adhering to thewall of the translucent hermetic vessel. On the other hand, in the caseof the mercury-containing lamp, an atom density ratio A(%) of SiO₂ ofabout 68% or lower is acceptable.

On the other hand, when the atom density ratio A(%) is lower than 2.5%,the operation and advantage of the metal halide lamp are not remarkablydifferent from those of the metal halide lamp with the atom densityratio falling within the range prescribed in this embodiment. However,if the atom density ratio A(%) of SiO₂ is reduced to such a low value,the manufacture cost of the metal halide lamp increases significantlyand the manufacture thereof becomes extremely difficult. Therefore, suchan atom density ratio is not acceptable. Preferably, the atom densityratio falls within the range expressed by the formula (9). That is, asfar as it falls within the range, even if, instead of mercury, a halideof a metal, such as Zn, which increases the lamp voltage is sealed as asecond halide together with the halide of a light-emitting metal, thelife of the metal halide lamp is not significantly reduced.2.5<A<20  (9)

In order to control the atom density ratio A(%) of SiO₂ at the surfaceof the tip ends of the electrodes as desired as described above, one ormore of exemplary measures described below may be advantageously,selectively used. However, this invention is not limited to use of aparticular measure.

-   1. The electrodes are sealed in the translucent hermetic vessel, and    the time required to process, that is, hermetically close the open    ends of the translucent hermetic vessel is shortened.-   2. The bulb section of the translucent hermetic vessel is shielded    from heat to prevent a high temperature of the bulb section during    the processing described above.-   3. The hermetic vessel is sealed containing a heavy gas at a high    pressure.-   4. The processing described above is conducted with the gas flowing.

The metal halide lamp according to the embodiment described in claim 8is the metal halide lamp described in claim 7 that is furthercharacterized in that the paired electrodes each have a part protrudinginto the discharge space which has a length of 1.9 mm or less.

According to this embodiment, an arrangement is prescribed to which arequired lamp characteristic is easily imparted and in which wear of theelectrodes is suppressed. In order to control the atom density ratioA(%) of SiO₂ at the surface of the tip ends of the electrodes as desiredas prescribed in claim 7, the parts protruding into the discharge spacecan be elongated to locate the tip ends of the electrodes away from thesealing parts. However, if such a measure is taken, there arises aproblem that it is difficult to provide a required lamp characteristic.

According to this embodiment, as far as the length of the protrudingparts of the electrodes is 1.9 mm or less as described above, a requiredlamp characteristic can be secured. However, when the length of theprotruding parts of the electrodes is 1.9 mm or less, the atom densityratio A(%) of SiO₂ at the surface of the tip ends of the electrodes ishigher than the upper limit of the formula (8) with a probability of40%. However, the measures in the above description concerning theembodiment described in claim 7 can be used to satisfy the formula (8),for example. As a result, wear of the electrodes can be effectivelysuppressed.

According to this embodiment, in order to provide a desired lampcharacteristic, the lamp voltage can be set to fall within a range of 25to 70 V.

The metal halide lamp according to the embodiment described in claim 9is the metal halide lamp described in any one of claims 1 to 8 that isfurther characterized in that the discharge medium contains a halide ofa light-emitting metal as a first halide, and one or more of halides ofMg, Co, Cr, Zn, Mn, Sb, Re, Ga, Sn, Fe, Al, Ti, Zr and Hf as a secondhalide.

According to this embodiment, an arrangement is prescribed in which thesecond halide, which serves to provide a lamp voltage instead ofmercury, is added to the first halide, which is a halide of alight-emitting metal. The second metal halide is characterized in thatit has a relatively high vapor pressure and emits relatively littlevisible light. Thus, selectively sealing an appropriate amount of secondhalide can increase the lamp voltage so as to fall within a desiredrange. Therefore, a required lamp power can be input with a relativelylow lamp current.

The metal halide lamp according to the embodiment described in claim 10is the metal halide lamp described in claim 9 that is furthercharacterized in that the second halide is a halide of Zn.

According to this embodiment, an arrangement is prescribed in which apreferred second halide is used. That is, Zn has a high vapor pressure,emits blue light and, therefore, is capable of color adjustment. Zn isavailable at a low cost in a required amount and is highly safety.

A metal halide lamp lighting device according to the embodimentdescribed in claim 11 is characterized in that the metal halide lamplighting device comprises: a metal halide lamp according to any one ofclaims 1 to 10; and a lighting circuit in which a maximum lamp power atthe start of lighting within 4 seconds after the metal halide lamp isturned on is two to four times higher than a lamp power in a stablestate.

This embodiment relates to a metal halide lamp suitable for anautomotive headlamp.

According to this embodiment, since the lighting device is controlled tomake the maximum lamp power within 4 seconds after the metal halide lampis turned on 2 to 4 times higher than the lamp power in the stablestate, the rising of luminous flux within 4 seconds after the lamp isturned on can be more rapid. In this invention, alternating-currentlighting or direct-current lighting may be adopted. In the case of thealternating-current lighting, a low-frequency rectangularalternating-current voltage can be applied to turn on the metal halidelamp to effectively suppress the occurrence of an acoustic resonance.

Furthermore, the lighting circuit can be designed to have a no-loadoutput voltage of 200 V or lower. The lamp voltage of the metal halidelamp used in this invention is lower than that of the mercury-containinglamp, and therefore, the no-load output voltage of the lighting circuitcan be 200 V or lower. This enables downsizing of the lighting circuit.Here, in the case of the mercury-containing lamp, a no-load outputvoltage of about 400 V is required.

An automotive headlamp apparatus according to the embodiment describedin claim 12 is characterized in that the automotive headlamp apparatuscomprises: an automotive headlamp apparatus main unit; a metal halidelamp according to any one of claims 1 to 10 which is installed in theautomotive headlamp apparatus main unit with the axis of a dischargevessel thereof being aligned with an optical axis of the automotiveheadlamp apparatus main unit; and a lighting circuit in which a maximumlamp power at the start of lighting within 4 seconds after the metalhalide lamp is turned on is two to four times higher than a lamp powerin a stable state.

Since the automotive headlamp apparatus of this embodiment has the metalhalide lamp described in any of claims 1 to 10 as a light source, itprovides a rapid rising of luminous flux and is safety. In addition,since the metal halide lamp contains no mercury, which applies asignificant load to the environment, the automotive headlamp apparatusis highly preferable from an environmental viewpoint. Here, the“automotive headlamp apparatus main unit” refers to the whole of theautomotive headlamp apparatus excluding the metal halide lamp and thelighting circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing variations of lamp currents of a mercury-freelamp and a mercury-containing lamp after the lamps are turned on;

FIG. 2 is a graph showing variations of electrode temperatures thereof;

FIG. 3 is a graph showing variations of vapor pressures thereof;

FIG. 4 is a graph showing rising characteristics of luminous flux of aconventional mercury-free lamp and a mercury-free lamp according to thepresent invention at the time of turn-on;

FIG. 5 is a front view of a metal halide lamp according to an embodimentdescribed in claim 1;

FIG. 6 is an enlarged view of an essential part of the metal halide lampturned off;

FIG. 7 is a graph showing how the starting light emission ratio maximumvalue varies when the ratio H/C of the amount H of the halides depositedon the electrodes to the inner volume C of the hermetic vessel and thevalue of the formula (1) are varied;

FIG. 8 is a graph showing how the starting light emission ratio maximumvalue varies when the melting point T of the halides and the value ofthe formula (1) are varied;

FIG. 9 is a graph showing how the starting light emission ratio maximumvalue varies when the ratio R of the maximum lamp power at the start oflighting to the lamp power in a stable state and the value of theformula (1) are varied;

FIG. 10 is a front view of essential parts of a metal halide lampaccording to an embodiment described in claim 2;

FIG. 11 is a cross sectional view of the middle part of the metal halidelamp;

FIG. 12 is a graph showing relationships between the rising of luminousflux 4 seconds after lamp's turning on and the value of(L_(A−H))³×C_(T)/B_(W) and between the relative lamp life and the valueof (L_(A−H))³×C_(T)/B_(W) in the case where a lamp power 2.4 timeshigher than the lamp power in the stable state is input;

FIG. 13 is an enlarged front view of essential parts of a metal halidelamp according to embodiments described in claims 3 to 5;

FIG. 14 is an enlarged front view of essential parts of a metal halidelamp according to an embodiment described in claim 6;

FIG. 15 is a graph showing how the electrode life and the distance to anarc-originating point vary when an electrode tip angle Q is varied inthe example shown in FIG. 14;

FIG. 16 is an enlarged front view of essential parts of a metal halidelamp according to a modification of the embodiment described in claim 6;

FIG. 17 is an enlarged front view of essential parts of a metal halidelamp according to another modification of the embodiment described inclaim 6;

FIG. 18 is a graph showing how a 2000-hour luminous flux maintenancefactor varies when an atom density ratio A of SiO₂ at the surface of thetip end of the electrode varies in the embodiment described in claim 7;

FIG. 19 is a graph showing a relationship between the length of theprotruding part of the electrode and the atom density ratio A of SiO₂ atthe surface of the tip end of the electrode in the case where one end ofthe hermetic vessel made of quartz glass is sealed while sealing thereinthe electrodes without using any particular means to reduce the atomdensity ratio of SiO₂;

FIG. 20 is a front view illustrating another example according to theembodiments described in claims 1, 2 and 7;

FIG. 21 is a circuit diagram of a metal halide lamp lighting deviceaccording to an embodiment described in claim 11;

FIG. 22 is a perspective view of an automotive headlamp apparatusaccording to an embodiment described in claim 12; and

FIG. 23 is an enlarged front view of essential parts of a conventionalmercury-containing lamp turned off.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, embodiments described in the claims will be describedwith reference to the drawings.

<Embodiment Described in Claim 1>

This embodiment will be described with reference to FIGS. 5 and 6. Inthe drawings, a metal halide lamp MHL comprises a discharge vessel 1, asealed metal foil 2, an externally introduced line 3 and a dischargemedium.

The discharge vessel 1 comprises a translucent hermetic vessel 1 a and apair of electrodes 1 b, 1 b. The hermetic vessel 1 a is shaped into ahollow spindle and has a pair of elongated sealing parts 1 a 1 formedintegrally therewith at both ends. The inside of the hermetic vessel 1 aprovides an elongated and substantially cylindrical discharge space 1 c.The volume of the discharge space 1 c of the hermetic vessel 1 a, thatis, the inner volume thereof is denoted by C in terms of cc.

The paired electrodes 1 b, 1 b are held at predetermined positions withtheir base ends embedded in the sealing parts 1 a 1 and their tip endsprotruding into the discharge space 1 c. The base portion of eachelectrode 1 b is welded to one end of the sealed metal foil 2 in thesealing part 1 a 1.

The sealed metal foil 2 is made of molybdenum and hermetically sealed insealing part 1 a 1 of the hermetic vessel 1 a.

The externally introduced line 3 has a tip end welded to the sealedmetal foil 2 in the sealing part 1 a 1.

The discharge medium is composed of halides and xenon and sealed in thedischarge space 1 c of the hermetic vessel 1 a. When the metal halidelamp is on, an excess of the halide is in the liquid phase and depositedon the inner wall of the hermetic vessel 1 a. Reference numeral 4 inFIG. 6 denotes the halides in the liquid phase. The halides sealed inthe hermetic vessel 1 a are a first metal halide, which is a halide of alight-emitting metal, and a second halide, which has a relatively highvapor pressure. The melting point of the mixture of the halides isdenoted by T in terms of K. The first halide contains a halide of sodiumNa and at least one of halides of scandium Sc and rare earth metals. Asignificant amount of the sealed halides is deposited on the innersurface of the hermetic vessel 1 a in the form of the halide 4 in aliquid state, and the amount of the halides deposited on the electrodes1 b is reduced. This is because the shape of the end parts of thehermetic vessel 1 a surrounding the electrodes 1 b is modified from theone indicated by the dashed line to the one indicated by the solid line,thereby bringing the inner wall close to the electrodes, the shape ofthe parts of the hermetic vessel 1 a in which the electrode are embeddedis modified from the one indicated by the dotted line to the oneindicated by the solid line, thereby eliminating the wedge-shapedclearance, and, although not shown, the length of the discharge space isreduced. Xenon is sealed in the vessel at 3 atmospheres or higher.

The metal halide lamp according to this embodiment is turned on at aratio R of a starting maximum lamp power to a stable lamp power. In thefollowing, examples and a comparison example will be described. Thecomparison example is related with a conventional mercury-free lamp. The“starting light emission ratio maximum value” refers to a ratio of aninstantaneous maximum luminous flux within 2 seconds after the lamp isturned on to a luminous flux in a stable state.

(EXAMPLE 1)

Discharge Vessel

The hermetic vessel 1 a was made of quartz glass and had an outerdiameter of 6 mm, an inner diameter of 3 mm, an inner volume of 0.03 cc,and a discharge space length of 6.6 mm.

The electrodes 1 b were made of tungsten, the shaft parts thereof had adiameter of 0.4 mm, and the distance between the electrodes was 4.2 mm.

Discharge Medium

The halides used were ScI₃, NaI and ZnI₂ in a relation ofScI₃−NaI−ZnI₂=1.2 mg, the amount H of the halides deposited on theelectrodes was 0.03 mg, and the melting point T thereof was 650 K.

Xenon Gas was at 10 Atmospheres.

The maximum lamp power at the start of lighting was 105 W, and the lamppower in a stable state was 35 W.

The value X of the formula (1), that is, (H/C)×[R/(T/500)⁶] was 0.62.

The starting light emission ratio maximum value E was 105% (indicated bythe curve C in FIG. 4), and no visible orange light was emitted.

(EXAMPLE 2)

Discharge Medium

The amount H of the halides deposited on the electrodes was 0.15 mg, andthe melting point T thereof was 750 K.

The other points were the same as those in Example 1.

The value X of the formula 1 was 1.31.

The starting light emission ratio maximum value E was 105%, and novisible orange light was emitted.

(Example 3)

Discharge Medium

The amount H of the halides deposited on the electrodes was 0.15 mg, andthe melting point T thereof was 650 K.

The other points were the same as those in Example 1.

The maximum lamp power at the start of lighting was 70 W.

The other points were the same as those in Example 1.

The value X of the formula 1 was 2.07.

The starting light emission ratio maximum value E was 60%, and novisible orange light was emitted.

(COMPARISON EXAMPLE)

Discharge Vessel

The hermetic vessel 1 a was made of quartz glass and had an outerdiameter of 6 mm, an inner diameter of 3 mm, an inner volume C of 0.03cc, and a discharge space length of 7.8 mm.

The electrodes 1 b were made of tungsten, the shaft parts thereof had adiameter of 0.4 mm, and the distance between the electrodes was 4.2 mm.

Discharge Medium

The halides used were ScI₃, NaI and ZnI₂ in a relation ofScI₃−NaI−ZnI₂=1.2 mg, the amount H of the halides deposited on theelectrodes was 0.22 mg, and the melting point T thereof was 650 K.

Xenon gas was at 10 atmospheres.

The maximum lamp power at the start of lighting was 105 W, the lamppower in a stable state was 35 W, and the ratio R of the maximum lamppower at the start of lighting to the lamp power in a stable state was3.

The value X of the formula (1) was 4.56.

The starting light emission ratio maximum value E was 160% (indicated bythe curve C in FIG. 4), and orange light was emitted.

Now, with reference to FIGS. 7 to 9, there will be described how thestarting light emission ratio maximum value E varies when the amount Hof the halides deposited on the electrodes, the inner volume C of thehermetic vessel, the melting point T of the halides and the ratio R ofthe maximum lamp power at the start of lighting to the lamp power in astable state are varied. In these drawings, the left-side vertical axisindicates the starting light emission ratio maximum value E, and theright-side vertical axis indicates value of the formula (1). In thesedrawings, the curve e is for the starting light emission ratio maximumvalue E, and the curve x is for the value X of the formula (1).

FIG. 7 shows how the starting light emission ratio maximum value and thevalue of the formula (1) vary when the ratio H/C of the amount H of thehalides deposited on the electrodes to the inner volume C of thehermetic vessel is varied. In this drawing, the horizontal axisindicates the ratio H/C of the amount H of the halides deposited on theelectrodes to the inner volume C of the hermetic vessel.

FIG. 8 shows how the starting light emission ratio maximum value and thevalue of the formula (1) vary when the melting point T of the halides isvaried. In this drawing, the horizontal axis indicates the melting pointT of the halides.

FIG. 9 shows how the starting light emission ratio maximum value and thevalue of the formula (1) vary when the ratio R of the maximum lamp powerat the start of lighting to the lamp power in a stable state is varied.In this drawing, the horizontal axis indicates the ratio R of themaximum lamp power at the start of lighting to the lamp power in astable state.

As can be seen from these drawings, the values of the formula (1)relatively approximate to experimental values, and thus, the formula (1)is appropriate.

<Embodiment Described in Claim 2>

This embodiment will be described with reference to FIGS. 10 and 11.While the metal halide lamp according to this embodiment is apparentlysimilar to that shown in FIG. 5, the lamp power B_(W) (W) in a stablestate, the thickness d_(A) (mm) of an arc, the minimum length L_(A−H)(mm) between a point in the arc having a maximum luminance and a pool ofthe discharge medium and the mass C_(T) (mg) of the discharge spacesection (having a length of 1) of the hermetic vessel are determined tosatisfy the formula (2) (5<(L_(A−H))³×C_(T)/B_(W)<28).

(EXAMPLE 4)

Discharge Vessel

The hermetic vessel 1 a was made of quartz glass and had an outerdiameter of 5 mm, an inner diameter of 2.2 mm, and a length of 6.5 mm,and the mass C_(T) of the discharge space section was 250 mg.

The electrodes 1 b were made of tungsten, the diameter of the tip endsthereof was 0.4 mm, the length of the protruding sections thereof was2.3 mm, the diameter d_(E) of the shafts parts was 0.4 mm, and thedistance between the electrodes was 4.2 mm.

Discharge Medium

The halides used were ScI₃, NaI and ZnI₂ in a relation ofScI₃−NaI−ZnI₂=0.2 mg.

Xenon gas was at 6 atmospheres.

The lamp power B_(W) in a stable state was 35 W, and the minimum lengthL_(A−H) was 1.4 mm.(L _(A−H))³ ×C _(T) /B _(W)=19.60

Now, concerning Example 4, the rising of luminous flux 4 seconds afterthe lamp is turned on and the life of the electrodes in the case wherethe variable term ((L_(A−H))³×C_(T)/B_(W)) of the formula (2) is changedwill be described with reference to FIG. 12. In FIG. 12, the horizontalaxis indicates the value of (L_(A−H))³×C_(T)/B_(W), and the verticalaxis indicates the rising of luminous flux (%) at the time of 2.4-timeinput and the relative lamp life (%). The curve r is for the rising ofluminous flux, and the curve 1 is for the electrode life. Here, the“rising of luminous flux at the time of 2.4-time input” described abovemeans the rising of luminous flux 4 seconds after the lamp is turned onin the case where a lamp power 2.4 times higher than the lamp power inthe stable state is input. In addition, the “relative lamp life” is arelative value of lamp life assuming that the longest lamp life data is100%.

As can be seen from this drawing, if the value of the formula (2) islower than the lower limit thereof, the value of the rising of luminousflux becomes extremely high, and the electrode life is extremelydeteriorated. And, if the value of the formula (2) is higher than theupper limit thereof, the value of the rising of luminous flux is lowerthan 70%.

<Embodiments Described in Claims 3 to 5>

These embodiments will be described with reference to FIG. 13. Accordingto these embodiments, the electrode 1 b has a maximum-diameter section 1b 2 composed of a tungsten coil at a short distance from the tip end 1 b1 of the electrodes 1 b. The tip end 1 b 1 has a diameter of A_(E) (mm),the maximum-diameter section 1 b 2 has a diameter of B_(E) (mm), thepart protruding into the discharge space 1 c has an average diameter ofD_(E) (mm), and the part 1 b 4 embedded in the sealing part has anaverage diameter of C_(E) (mm).

The embodiment according to claim 3 satisfies the formulas (6) and (7),the embodiment according to claim 4 satisfies the formula (8), and theembodiment according to claim 5 satisfies the formulas (9) and (10).

(EXAMPLE 5)

Discharge Vessel

The hermetic vessel 1 a was made of quartz glass and had an outerdiameter of 6 mm and an inner diameter of 3.0 mm.

The electrodes 1 b were made of tungsten. The diameter A_(E) of the tipend 1 b 1 thereof (10% from the tip) was 0.3 mm, the diameter B_(E) ofthe maximum-diameter section 1 b 2 was 0.5 mm, the average diameterd_(E) of the protruding part was 0.42 mm, the average diameter C_(E) ofthe embedded part 1 b 4 was 0.3 mm and the distance between theelectrodes was 4.2 mm.

Discharge Medium

The halides used were ScI₃, NaI and ZnI₂ in a relation ofScI₃−NaI−ZnI₂=0.2 mg.

Xenon gas was at 6 atmospheres.

The lamp power in a stable state was 35 W.

<Embodiment Described in Claim 6>

This embodiment will be described with reference to FIG. 14. Accordingto this embodiment, the electrode 1 b has a maximum-diameter section 1 b2 composed of a tungsten coil at a short distance from the tip end 1 b1, and an electrode tip angle Q between a line parallel to the axis ofthe hermetic vessel 1 a and a line connecting the tip end of theelectrode to a shoulder of the maximum-diameter section 1 b 2 near thetip end falls within a range of 24 to 43 degrees.

Now, with reference to FIG. 15, there will be described how the life ofthe electrode and the distance to an arc-originating point vary when theelectrode tip angle Q is varied. In this drawing, the horizontal axisindicates the electrode tip angle Q (°), and the left-side vertical axisindicates the life (h) of the electrode, and the right-side verticalaxis indicates the distance (mm) to the arc-originating point. The“distance to an arc-originating point” refers to the distance from thetip of the electrode to the point where an arc is originated. The curve1 is for the life of the electrode, and the curve d is for the distanceto the arc-originating point.

As can be seen from this drawing, when the electrode tip angle Q fallswithin the range of 24 to 43 degrees, the distance to thearc-originating point is 0, and the life of the electrode is long.

Now, with reference to FIGS. 16 and 17, modifications of the embodimentdescribed in claim 6 will be described.

First, in a modification shown in FIG. 16, the tip end 1 b 1 of theelectrode 1 b has a semispherical shape. In such a case, assuming thatthe tip end of the electrode is planar, the electrode tip angle Q ismeasured as in the case shown in FIG. 14 and set to fall within therange of 24 to 43 degrees.

In a modification shown in FIG. 17, the electrode 1 b has alarge-diameter section 1 b 3 formed at the tip end thereof. In such acase, the electrode tip angle Q_(E) between a line s1 and the axis fallswithin the range of 24 to 43 degrees, the line s1 being the first, amonglines extending from the shoulder of the tip end of the electrode, tointersect with the circumference of the large-diameter section 1 b 3when being rotated toward the axis.

<Embodiment Described in Claim 7>

While the metal halide lamp according to this embodiment is apparentlysimilar to that shown in FIG. 5, it is configured so that the atomdensity ratio A (%) of SiO₂ at the surface of the tip end of theelectrode satisfies the formula (8).

(EXAMPLE 6)

Discharge Vessel

The hermetic vessel 1 a was made of quartz glass, and the bulb section 1a 1 had an outer diameter of 6 mm and an inner diameter of 3 mm.

The electrodes 1 b were made of tungsten, the shaft parts thereof had adiameter of 0.4 mm, and the distance between the electrodes was 4.2 mm.

Discharge Medium

The halides used were SCI₃, NaI and ZnI₂ in a relation ofScI₃−NaI−ZnI₂=1.2 mg.

Xenon gas was at 6 atmospheres.

Sealing was conducted in such a manner that, in a pressure box having anatmosphere kept at 3 atmospheres, xenon at −44° C. was sealed in thehermetic vessel 1 a, the sealing parts made of quartz glass were heatedand molten by a laser, and pinch sealing was performed with a pincher.

For the resulting metal halide lamp, the atom density ratio A of SiO₂ atthe surface of the tip end of the electrode was 0.5%. This is becausescattering of SiO₂ was adequately suppressed due to the high pressuresealing of xenon.

Now, with reference to FIG. 18, there will be described how the2000-hour luminous flux maintenance factor varies when the atom densityratio A of SiO₂ at the surface of the tip end of the electrode of themercury-free lamp is varied. In this drawing, the horizontal axisindicates the atom density ratio A (%) of SiO₂, and the vertical axisindicates the 2000-h luminous flux maintenance factor (%).

As can be seen from this drawing, when the atom density ratio A is lowerthan 43%, an improved luminous flux maintenance factor is provided.

(EXAMPLE 7)

Discharge Vessel

The cross sectional area B of the sealing part 1 a 2 at the joint to thebulb section 1 a 1 was 5.34 mm² (diameter: 2.7 mm).

Discharge Medium

The halides used were ScI₃, NaI and ZnI₂ in a relation ofScI₃−NaI−ZnI₂=0.9 mg.

Xenon gas was at 13.5 atmospheres.

The other points were the same as in Example 6.

<Embodiment Described in Claim 8>

This embodiment will be described with reference to FIG. 19. In thisdrawing, the horizontal axis indicates the length (mm) of the protrudingpart of the electrode, and the vertical axis indicates the atom densityratio (%) of SiO₂ at the surface of the tip end of the electrode. Thisgraph is made in the following manner. That is, quartz glass tubeshaving electrodes inserted therein are heated at portions to be sealedin an N₂ atmosphere to make the portions molten, and then the moltenportions are sealed with a pincher, thereby providing a plurality oftest pieces with the electrodes having different protruding lengths.Then, the atom density ratio of SiO₂ at the surface of the tip end ofthe electrodes is measured for the test pieces, and the measurements areused to plot the graph.

As can be seen from this drawing, if no particular means to reduce theatom density ratio of SiO₂ is used, when the length of the protrudingpart of the electrode is 1.9 mm or less, the atom density ratio ishigher than 43% for most test pieces. In such a case, measures describedconcerning the embodiment of claim 7 may be selectively used.

<Another Example of Embodiments Described in Claims 1 to 9>

Another embodiment will be described with reference to FIG. 20.According to this embodiment, a metal halide lamp similar to that shownin FIG. 5 is mounted on an automotive headlamp apparatus. That is, themetal halide lamp (MHL′) comprises a light-emitting tube (LT), an outerjacket (OT), a cap (B) and an insulation tube (IT).

The light-emitting tube (LT) is configured the same as the metal halidelamp (MHL′) shown in FIG. 5. The parts same as those in FIG. 5 areassigned the same reference numerals, and the descriptions thereof areomitted.

The outer jacket (OT) can block the ultraviolet rays. It houses thelight-emitting tube (LT) therein and is fixed to the sealing parts (1 a1) at the both ends. However, it is not hermetically sealed butcommunicated with the outside air.

The cap (B) serves both to support the light-emitting tube (LT) and theouter jacket (OT) and to electrically interconnect the pair ofelectrodes (1 b), (1 b) of the light-emitting tube (LT). That is, one ofthe sealing parts (1 a 1) of the light-emitting tube (LT) is secured tothe cap (B), and an external lead wire (3) drawn from the other sealingpart extends parallel to the outer jacket (OT) and then is introducedinto the cap (B) and connected to a terminal (not shown)

The insulation tube (IT) covers the external lead wire (3).

<Embodiment Described in Claim 10>

This embodiment will be described with reference to FIG. 20. In thisdrawing, a metal halide lamp lighting device comprises a lightingcircuit (OC) and a metal halide lamp (MHL).

The lighting circuit (OC) comprises a direct-current power supply (11),a chopper (12), control means (13), lamp current detecting means (14),lamp voltage detecting means (15), an igniter (16) and a full-bridgeinverter (17).

The direct-current power supply (11) is to supply a direct current powerto the chopper (12) described later and may be a battery or rectifieddirect-current power supply. In the automotive application, a battery istypically used. Alternatively, it may be a rectified direct-currentpower supply that rectifies an alternating current. In any case,smoothing can be conducted with an electrolytic capacitor (11 a)connected in parallel as required.

The chopper (12) is a DC/DC converter circuit that converts adirect-current voltage applied by the direct-current power supply (11)into a direct-current voltage of a required value, and determines thevalue of the output voltage to be applied to the metal halide lamp (MHL)through the full-bridge inverter (17) described later. If the voltage ofthe direct-current power supply is lower than the required outputvoltage, a booster chopper is used. On the other hand, if the voltage ishigher than the required output voltage, a step-down chopper is used.

The control means (13) incorporates a microcomputer having a programmedtemporal control pattern and controls the chopper (12). For example, thecontrol means (13) controls the chopper (12) in such a manner that,immediately after the metal halide lamp is turned on, a lamp currentthree or more times higher than a rated lamp current is flowed from thechopper (12) to the metal halide lamp (MHL) via the full-bridge inverter(17), and then with the lapse of time, the lamp current is graduallyreduced to the rated lamp current. Furthermore, the control means (13)receives feedback of detection signals associated with the lamp currentand lamp voltage as described later, and thus, generates a constantpower control signal to perform constant power control on the chopper(12).

The lamp current detecting means (14) is inserted in series with thelamp via the full-bridge inverter (17) and detects a currentcorresponding to the lamp current to provide a control input to thecontrol means (13).

Similarly, the lamp voltage detecting means (15) is connected parallelto the lamp via the full-bridge inverter (17) and detects a voltagecorresponding to the lamp voltage to provide a control input to thecontrol means (13).

The igniter (16) is interposed between the full-bridge inverter (17) andthe metal halide lamp (MHL) and configured to apply a starting pulsevoltage on the order of 20 kV to the metal halide lamp (MHL) whenturning on the lamp.

The full-bridge inverter (17) comprises a bridge circuit (17 a)consisting of four MOSFETs (Q1), (Q2), (Q3) and (Q4), a gate drivecircuit (17 b) that alternately switches between the MOSFETs (Q1) and(Q3) and the MOSFETs (Q2) and (Q4) in the bridge circuit (17 a), and apolarity inverting circuit (17 c). The full-bridge inverter (17)converts the direct current voltage from the chopper (12) into arectangular low-frequency alternating current voltage by the switchingand applies the resulting voltage to the metal halide lamp (MHL) to turnon the lamp with the low-frequency alternating current.

If the metal halide lamp (MHL) is turned on with the rectangularlow-frequency alternating current by the lighting circuit (OC) in thisway, the metal halide lamp produces a required luminous flux immediatelyafter it is turned on. Thus, 25% of the rated luminous flux can beattained 1 second after the power-on and 80% of the rated luminous fluxcan be attained 4 seconds after the power-on, which are requirements ofthe automotive headlamp.

<Embodiment Described in Claim 12>

This embodiment will be described with reference to FIG. 8. In thisdrawing, an automotive headlamp apparatus (HL) comprises an automotiveheadlamp apparatus main unit (21), a pair of lighting circuits (OC) anda pair of metal halide lamps (MHL′).

The automotive headlamp apparatus main unit (21) comprises a fronttransparent panel (21 a), reflectors (21 b), (21 c), a lamp socket (21d) and a fixture (21 e).

The front transparent panel (21 a) is contoured to the shape of theouter surface of the automobile and has required optical means, forexample, a prism.

Each of the reflectors (21 b), (21 c) is provided for each metal halidelamp (MHL′) and configured to provide required light distributioncharacteristics.

The lamp socket (21 d) is connected to an output terminal of thelighting circuit (OC) and is mounted in a cap (21 d) of the metal halidelamp (MHL′).

The fixture (21 e) is means for fixing the automotive headlamp apparatusmain unit (21) to the automobile at a predetermined position.

The metal halide lamp (MHL′) has the configuration described in claim 5shown in FIG. 20. The lamp socket (21 d) is mounted in the cap andconnected thereto.

In this way, the two-bulb metal halide lamp (MHL′) is mounted in theautomotive headlamp apparatus main unit (21), resulting in the four-bulbautomotive headlamp apparatus (HL) The light emitting parts of eachmetal halide lamp (MHL′) are located generally at focal points of thereflectors (21 b), (21 c) of the automotive headlamp apparatus main unit(21).

The lighting circuits (OC), which have the circuit arrangement shown inFIG. 21, are housed in metallic vessels (22) and energize the respectivemetal halide lamps (MHL′) to turn them on.

INDUSTRIAL APPLICABILITY

According to the embodiment described in claim 1, there is provided ametal halide lamp comprising: a discharge vessel having an inner volumeof C (cc); and a discharge medium containing xenon gas at 3 atmospheresor higher, a halide of sodium Na, and at least one of halides ofscandium Sc and rare earth metals, the melting point of the halidesbeing T (K), wherein the metal halide lamp is kept on with a lamp powerof 50 W or lower in a stable state, and the formula (1) is satisfied:(H/C)×[R/(T/500)⁶]<3.11  (1),where the amount of the halide deposited on the electrodes is H (mg),and the ratio of a maximum lamp power at the start of lighting to thelamp power in the stable state is R, whereby mercury is substantiallyeliminated from the lamp out of consideration to the environment, arapid rising of luminous flux is achieved, and the instantaneous intenselight emission within 2 seconds after the lamp is turned on issuppressed.

According to the embodiment described in claim 2, there is provided ametal halide lamp comprising a discharge vessel and a discharge medium,wherein the metal halide lamp is kept on with a lamp power of 50 W orlower in a stable state, a lamp power 2.2 or more times higher than thelamp power in the stable state is supplied to the lamp during a periodof 10 seconds after the lamp is turned on, 60% or more of the luminousflux in the stable state is achieved 4 seconds after the lamp is turnedon, and the formula (2) is satisfied:5<(L _(A−H))³ ×C _(T) /B _(W)<28  (2),where the lamp power in the stable state is B_(W) (W), a minimum lengthbetween a point in an arc having a maximum luminance and a pool of thedischarge medium in the liquid phase is L_(A−H) (mm), and the mass ofthe discharge space section of the hermetic vessel is C_(T) (mg),whereby mercury is substantially eliminated from the lamp out ofconsideration to the environment, and the rising of luminous flux isremarkably improved.

According to the embodiment described in claim 3, since the electrodeseach have an average diameter of C_(E) (mm) in a section embedded in thehermetic vessel and have a maximum-diameter section in a part protrudinginto the discharge space, the average diameter of the protruding part isD_(E) (mm), and the formulas (3) and (4) are satisfied:C_(E<D) _(E)  (3), andD _(E) −C _(E)>0.05  (4)there is provided a metal halide lamp having a rapid rising of luminousflux and improved in efficiency and life.

According to the embodiment described in claim 4, since the maximumdiameter of the part of each electrode protruding into the dischargespace is B_(E) (mm), the average diameter for the distal 10% thereof isA_(E) (mm), and the formula (5) is satisfied:A_(E)<B_(E)  (5),there is provided a metal halide lamp in which displacement of a cathodespot is suppressed and the light distribution characteristic isprevented from fluctuating.

According to the embodiment described in claim 5, since the electrodeseach have an average diameter of C_(E) (mm) in a section embedded in thehermetic vessel, the maximum diameter of the part of each of the pairedelectrodes protruding into the discharge space is B_(E) (mm), theaverage diameter for the distal 10% thereof is A_(E) (mm), the averagediameter of the protruding part being D_(E) (mm), and the formulas (3)and (6) are satisfied:C_(E)<D_(E)  (3), andA _(E) <D _(E) <B _(E)  (6),there is provided a metal halide lamp in which displacement of a cathodespot is suppressed and the light distribution characteristic isprevented from fluctuating.

According to the embodiment described in claim 6, since each electrodehas a large-diameter section at a short distance from the tip end, andan angle Q_(E) (°) between the axis of the electrode and a line drawnfrom a shoulder of the tip end to pass through an outermost point of thelarge-diameter section satisfies the formula (7):24≦Q _(E)≦43  (7),there is provided a metal halide lamp in which displacement of a cathodespot is suppressed and the light distribution characteristic isprevented from fluctuating.

According to the embodiment described in claim 7, there is provided ametal halide lamp suitable for the automotive headlamp comprising: adischarge vessel for which the atom density ratio A (%) of SiO₂ at thesurface of the tip ends of the electrodes satisfies the formula (8):2.5<A<43  (8); anda discharge medium containing xenon gas at 3 atmospheres or higher andat least one of halides of sodium Na, scandium Sc and a rare earthmetal, wherein the metal halide lamp is kept on with a lamp power of 50W or lower in a stable state, a period in which a power 2.0 or moretimes higher than the lamp power in the stable state is input isprovided immediately after the lamp is turned on, mercury issubstantially eliminated from the lamp out of consideration to theenvironment, a rapid rising of luminous flux is achieved, wear of theelectrodes is reduced, the occurrence of various defects due to the wearof the electrodes is suppressed, and thus, the metal halide lamp isimproved in reliability.

According to the embodiment described in claim 8, since the electrodeseach have a part protruding into the discharge space which has a lengthof 1.9 mm or less, there is provided a metal halide lamp which has along life and is suitable for the automotive headlamp.

According to the embodiment described in claim 9, since the dischargemedium contains one or more of halides of Mg, Co, Cr, Zn, Mn, Sb, Re,Ga, Sn, Fe, Al, Ti, Zr and Hf as a second halide, which serves as amedium for providing a lamp voltage, there is provided a metal halidelamp which can be adequately used for various applications including theautomotive headlamp with using substantially no mercury, which applies asignificant load to the environment.

According to the embodiment described in claim 10, since the secondhalide is a halide of Zn, which has a high vapor pressure, emits bluelight and, therefore, is capable of color adjustment, there is providedan inexpensive and safe metal halide lamp.

According to the embodiment described in claim 11, there is provided ametal halide lamp lighting device having the advantages according toclaims 1 to 10.

According to the embodiment described in claim 12, there is provided anautomotive headlamp apparatus having the advantages according to claims1 to 10.

1. A metal halide lamp, comprising: a discharge vessel having a hermeticvessel which is fire resistant and translucent and has a discharge spacetherein, and a pair of electrodes provided at opposite ends of thedischarge space in the hermetic vessel with facing each other at adistance of 5 mm or less, the inner volume of the hermetic vessel beingC in terms of cc; and a discharge medium substantially containing nomercury, sealed in the hermetic vessel, and containing xenon gas at 3atmospheres or higher, a halide of sodium Na, and at least one ofhalides of scandium Sc and rare earth metals, the melting point of thehalides being T in terms of K, in a stable state, the metal halide lampis kept on with a lamp power of 50 W or lower, and the formula (1) issatisfied:(H/C)×[R/(T/500)⁶]<3.11  (1), where the amount of the halide depositedon the electrodes when the lamp is off is denoted by H in terms of mg,and the ratio of a maximum lamp power at the start of the lighting tothe lamp power in the stable state is denoted by R.
 2. A metal halidelamp, comprising: a discharge vessel having a hermetic vessel which isfire resistant and translucent and has a discharge space therein, and apair of electrodes sealed at opposite ends of the discharge space in thehermetic vessel with facing each other at a distance of 5 mm or less;and a discharge medium substantially containing no mercury, sealed inthe hermetic vessel, and containing a halide of a light-emitting metaland an inert gas, and in a stable state, the metal halide lamp is kepton with a lamp power of 50 W or lower, during a period of 10 secondsafter the lamp is turned on, a lamp power 2.2 or more times higher thanthe lamp power in the sable state is supplied to the lamp, 60% or moreof the luminous flux in the stable state is achieved 4 seconds after thelamp is turned on, and the formula (2) is satisfied:5<(L _(A−H))³ ×C _(T) /B _(W)<28  (2), where the lamp power in thestable state is B_(W) (W), a minimum length between a point in an archaving a maximum luminance and a pool of the discharge medium in theliquid phase is L_(A−H) (mm), and the mass of the discharge spacesection of the hermetic vessel is C_(T) (mg).
 3. The metal halide lampaccording to claim 2, wherein the paired electrodes each have an averagediameter of C_(E) (mm) in a section embedded in the hermetic vessel andhave a maximum-diameter section in a part protruding into the dischargespace, the average diameter of the protruding part being D_(E) (mm), andthe formulas (3) and (4) are satisfied:C_(E)<D_(E)  (3), andD _(E) −C _(E)>0.05  (4).
 4. The metal halide lamp according to claim 2,wherein the maximum diameter of the part of each of the pairedelectrodes protruding into the discharge space is B_(E) (mm), theaverage diameter for the distal 10% thereof is A_(E) (mm), and theformula (5) is satisfied:A_(E)<B_(E)  (5).
 5. The metal halide lamp according to claim 2, whereinthe paired electrodes each have an average diameter of CE (mm) in asection embedded in the hermetic vessel, the maximum diameter of thepart of each of the paired electrodes protruding into the dischargespace is B_(E) (mm), the average diameter for the distal 10% thereof isA_(E) (mm), the average diameter of the protruding part being D_(E)(mm), and the formulas (3) and (6) are satisfied:C_(E)<D_(E)  (3), andA _(E) <D _(E) <B _(E)  (6).
 6. The metal halide lamp according to claim2, wherein each of the paired electrodes has a large-diameter section ata short distance from the tip end, and an angle Q_(E) (°) between theaxis of the electrode and a line drawn from a shoulder of the tip end topass through an outermost point of the large-diameter section satisfiesthe formula (7):24≦Q _(E)≦43  (7).
 7. A metal halide lamp, comprising: a dischargevessel having a hermetic vessel in which is made of quartz glass and hasa discharge space therein, and a pair of electrodes provided at oppositeends of the discharge space in the hermetic vessel with facing eachother at a distance of 5 mm or less, the atom density ratio A (%) ofSiO₂ at the surface of the tip ends of the electrodes satisfying theformula (8):2.5<A<43  (8), a discharge medium substantially containing no mercury,sealed in the hermetic vessel, and containing xenon gas at 3 atmospheresor higher and at least one of halides of sodium Na, scandium Sc and arare earth metal, in a stable state, the metal halide lamp is kept onwith a lamp power of 50 W or lower, and a period in which a power two ormore times higher than the lamp power in the stable state is input isprovided immediately after the lamp is turned on.
 8. The metal halidelamp according to claim 7, wherein the paired electrodes each have apart protruding into the discharge space which has a length of 1.9 mm orless.
 9. The metal halide lamp according to any one of claims 1 to 8,wherein the discharge medium contains a halide of a light-emitting metalas a first halide, and one or more of halides of Mg, Co, Cr, Zn, Mn, Sb,Re, Ga, Sn, Fe, Al, Ti, Zr and Hf as a second halide.
 10. The metalhalide lamp according to claim 9, wherein the second halide is a halideof Zn.
 11. A metal halide lamp lighting device, comprising: a metalhalide lamp according to any one of claims 1 to 8; and a lightingcircuit in which a maximum lamp power at the start of lighting within 4seconds after the metal halide lamp is turned on is two to four timeshigher than a lamp power in a stable state.
 12. An automotive headlampapparatus, comprising: an automotive headlamp apparatus main unit; ametal halide lamp according to any one of claims 1 to 8 which isinstalled in the automotive headlamp apparatus with the axis of adischarge vessel thereof being aligned with an optical axis of theautomotive headlamp apparatus main unit; and a lighting circuit in whicha maximum lamp power at the start of lighting within 4 seconds after themetal halide lamp is turned on is two to four times higher than a lamppower in a stable state.